Cutting out irrregular parts of a mesh out
Hope this helps:

You can try centering the mesh in an axis (X or Y most commonly) and then use numpad 1 or numpad 3 to get an isometric view of the mesh along that axis. (Depending on your blender config, you might need to also press numpad 5)

Then you can press z and tab to go into wireframe mode and edit mode respectively.

Once there, you can select whatever vertices are not needed (To select vertices you can use the hotkey ctrl+shift+tab and select vertices in the popup menu) and delete them. Then press tab again to go back to object mode and add a mirror modifier to mirror the whole "correct" side of the mesh along the axis you need. Fiddle around with the merge and clipping options to get whatever desired effect you were looking for.